Honors Biology Unit 11 Review: Evolution - Mechanisms and Evidence

Evolution by natural selection explains how populations change over generations. Charles Darwin and Alfred Russel Wallace reached the idea independently, and Darwin published the full argument in On the Origin of Species in 1859, drawing on his voyage on HMS Beagle and the Galápagos finches.

Natural selection occurs when individuals vary in heritable traits and those traits affect reproductive success. Producing more offspring than the environment can support can create competition, but it is not required for selection. Over generations, alleles associated with greater reproductive success can become more common.

Fitness means reproductive success, not strength. A powerful deer that never breeds has a fitness of zero. An adaptation is a heritable trait that raises fitness in a specific environment, so the same trait can help in one place and hurt in another. Dark peppered moths became common on soot-covered trees in industrial England, then declined after clean-air laws cut pollution.

Selection acts on individuals, but populations are what evolve. Exposure to an antibiotic does not make a bacterium mutate because it needs resistance. Resistance mutations can arise before or during exposure, and the drug favors resistant cells by killing or slowing susceptible ones.

Artificial selection works the same way, except humans choose which individuals breed. Every dog breed, from Chihuahua to Great Dane, descends from wolf ancestors.

Several independent lines of evidence point to the same conclusion, which is that living species share common ancestors.

Homologous structures share an evolutionary origin but can have different functions. Your arm, a bat's wing and a whale's flipper all contain a humerus, radius, ulna and carpals. Analogous structures, like bird wings and insect wings, have a similar function that evolved independently. Vestigial structures, such as the reduced pelvis inside a whale, are inherited from ancestors in which they had a different or more prominent function.

Vertebrate embryos all form a notochord, a tail and pharyngeal pouches. Older books call these structures "gill slits," but gills develop from pharyngeal structures in fish and some amphibians. In humans, pharyngeal structures contribute to parts of the ear, throat and other head and neck structures.

Deeper rock layers generally hold older fossils. Transitional fossils like Tiktaalik (fish to four-legged animals, about 375 million years ago) and Archaeopteryx (dinosaurs to birds) show intermediate forms. In biogeography, island species like Galápagos finches most resemble species on the nearest mainland.

Worked example. Cytochrome c is a protein of 104 amino acids found in nearly all oxygen-using organisms. Human and horse versions differ at 12 positions, so they are $\frac{104-12}{104} = \frac{92}{104} \approx 88\%$ identical. Human and yeast differ at about 44 positions, giving $\frac{60}{104} \approx 58\%$. Fewer differences can suggest a more recent common ancestor, but evolutionary rates can vary, so scientists compare multiple sources of evidence.

Population genetics measures evolution as a change in allele frequencies in a gene pool, the full set of alleles in a population. The Hardy-Weinberg principle says those frequencies stay constant if five conditions hold: no mutation, random mating, no natural selection, no gene flow, and a very large population (so no genetic drift). No real population meets all five, so Hardy-Weinberg is a baseline. A population that doesn't match its genotype predictions has departed from the model, but that alone does not prove its allele frequencies are changing.

For one gene with two alleles, $p + q = 1$ and $p^2 + 2pq + q^2 = 1$. Here $p^2$ is homozygous dominant, $2pq$ is heterozygous and $q^2$ is homozygous recessive.

Worked example. A population of 1,000 flowers is in Hardy-Weinberg equilibrium for a gene with two alleles. Of these flowers, 90 are white, the recessive phenotype ($aa$). So $q^2 = 90/1000 = 0.09$, which gives $q = \sqrt{0.09} = 0.3$ and $p = 1 - 0.3 = 0.7$. Heterozygotes are $2pq = 2(0.7)(0.3) = 0.42$, or 420 plants. Homozygous dominant plants are $p^2 = 0.49$, or 490. These add up to 490 + 420 + 90 = 1,000.

Some departures from these conditions can change allele frequencies. Nonrandom mating can change genotype frequencies without changing allele frequencies. Genetic drift is random change that affects small populations most. In a bottleneck, a disaster shrinks a population; northern elephant seals fell to about 20 animals in the 1890s and still have very low diversity. In the founder effect, a small group starts a new population, which is why Ellis-van Creveld syndrome is common among the Amish of Lancaster County, Pennsylvania. Gene flow moves alleles between populations and makes them more alike.

Under the biological species concept, a species is a group whose members can interbreed in nature and produce fertile offspring. Speciation can occur as populations diverge and reproductive barriers reduce gene flow. Those barriers do not always prevent every instance of interbreeding.

In allopatric speciation, a geographic barrier splits a population. For example, the formation of the Isthmus of Panama separated marine populations, and some snapping shrimp populations on opposite sides diverged into separate species. In sympatric speciation, populations split while living in the same area. In plants, polyploidy can contribute to sympatric speciation. Polyploid offspring have extra chromosome sets, which can reduce fertility when they cross with members of the parent population. Wheat is one polyploid crop.

Reproductive isolation keeps new species separate. Prezygotic barriers stop fertilization through different habitats, breeding times, courtship signals, mismatched reproductive parts, or sperm and eggs that can't fuse. Postzygotic barriers act after fertilization, when hybrids die early, are sterile, or have weak offspring. A mule, from a horse and a donkey, is healthy but sterile, so horses and donkeys stay separate species.

Macroevolution is change above the species level. Gradualism describes evolutionary change that accumulates relatively slowly over long periods. Punctuated equilibrium describes long periods of little change interrupted by relatively rapid change on a geological timescale, often associated with speciation. The fossil record shows both patterns. Adaptive radiation, like the more than a dozen Darwin's finch species descended from one ancestor, is one lineage splitting into many to fill open niches.